Process for metal coating cellulose triacetate



United States Patent 3,296,013 PROCESS FOR METAL COATING CELLULOSETRIACETATE Joseph L. Feeley, Rahway, N.J., assignor to Esso Research andEngineering Company, a corporation of Delaware No Drawing. Filed Oct.30, 1963, Ser. No. 319,976 6 Claims. (Cl. 117-60) This invention relatesto metal coated membranes. In particular, this invention relates tometal coated plastic film and a method of depositing a metal coating onsuch plastic film. More particularly, this invention relates to a methodof metal coating cellulose triacetate film and such metal coatedcellulose triacetate films.

Heretofore, the art has found it exceedingly difiicult to metal coatcellulose triacetate films. One method employed was to physicallyimpress finely divided metal powders or fine mesh metal sieves into thesurface of a partially polymerized resin or into a material wherein acompletely polymerized resin is bound in a thermoplastic resin matrix.plastic binder has been employed to cause the metal particles to adhereto a membrane surface. Methods which were tried and proved to beunsuccessful were the dyeing of the surface of the cellulose triacetatefilm with a dye containing redox groups or ion exchange groups. Suchgroups were to deposit catalyst cites upon the membrane for thesubsequent deposition of a continuous metal coating. Such coating doesnot adhere to the membrane. The impressing of metal into a partiallypolymerized surface or using a plastic binder to bind metal particles tothe surface of the triacetate film are unsatisfactory wherein the metalcoated film is to be used as an electrode or a resistance heater. Inorder to perform efficiently as a resistance heater or an electrode, acontinuous layer of conducting material essential to the transmission ofelectrons to or from all parts of the coated surface is necessary. It isdifficult to achieve this type of continuous coating by impressing themetal powders into the surface of the partially polymerized film or byusing a plastic binder to adhere the metal particle to the plastic film.

It has now been discovered that highly effective metal coated cellulosetriacetate film may be prepared by coating a thin film with a continuousmetal coating. The process involves treating the film first with aquaregia and then contacting the treated film with a silver salt solution.Heretofore, it has been extremely difiicult to metal coat cellulosetriacetate films with a thin continuous electrically conductive coatingof metal. By the instant process, the cellulose triacetate film may becoated with various metals. The process of this invention comprises thefirst step of treating the film with aqua regia'for about 10 to 20minutes, preferably 13 to 17 minutes and a second step of soaking thetreated film in a 20 to 30% solution of silver nitrate and a reducingagent for about to 20 minutes. The second step may be repeated 21 numberof times to insure that there is a continuous coating of the silvermetal on the triacetate film.

Once a continuous layer of silver is formed on the film, other materialscan be electrically deposited upon the silver by using the silversurface as a cathode in a conventional electrodeposition cell having anelectrolyte bath of the desired metal ions such as copper, the noblemetals such as platinum, gold, iridium, rhodium and palladium as well asthe transition metals such as nickel, chromium, iron and cobalt. Othermetals may be chemically exchanged for the silver to provide acontinuous metal coating of some metal other than silver. Other metalsthat may be chemically exchanged for the silver are metals such asplatinum, gold, iridium, rhodium and palladium.

In other embodiments, a thermo- The process of providing a metal surfaceon a triacetate film by exchanging the metal for silver comprises thesteps of (1) treating the triacetate fihn with aqua regia for 15minutes, (2) soaking the treated film in a 20 to 30% solution of silvernitrate and a reducing agent for about 6 minutes, (3) treating thetriacetate film in a 0.01 molar gold chloride solution, and (4) washingthe triacetate fil-m with a silver ion complexing agent in order toremove the excess silver ions. The second and third steps may berepeated a number of times to insure a continuous coating of the metalfilm.

The reducing agents which can be used in the practice of this inventioninclude hydroquinone, a solution of hydroquinone and gum arabic and ahypophosphite salt. The silver ion complexing agents include ammoniumhydroxide and thiosulfates such as sodium thiosulfate and potassiumthiosulfate.

A nickel coating may be applied by the same procedure as aforedescribedfor a silver coating. The acetate film is treated with aqua regia,silver nitrate and then ions of gold, platinum or palladium are appliedto the treated surface to establish catalyst cites for the deposition ofthe nickel. The film containing the catalyst cites is then contactedwith an aqueous solution containing nickel ions and a reducing agentsuch as a hypophosphite salt, particularly sodium or potassiumhypophosphite. This em.- bodiment is preferably carried out attemperatures between 60 to C.

The metal coated cellulose triacetate films prepared by this process maybe used as electrodes in a fuel cell or electrochemical cell and/or asdividers to divide a cell into two or more compartments or may be usedas the heating element in resistance heaters. That is, the electricalcurrent may be induced into the metal foil causing it to heat andradiate heat therefrom. Such heaters may be used in the building trade,that is, large sheets of metal coated cellulose triacetate may be placedbehind the walls to be used as radiators of heat. A metal coated film ofthis invention can serve as both an anode or cathode in a fuel cellwhich used either an acidic or basic electrolyte. It is, of course,obvious to those skilled in the art that when using an acid electrolytethat the metals deposited upon the cellulose triacetate film must benonreactive with the acid electrolyte. Therefore, metals such as nickeland copper could not be used. However, the noble metals would be veryuseful. The following examples are illustrative of the process of thisinvention and should not be construed as limitations upon the true scopeof this invention as set forth in the specification and claims.

Example 1 A square inch cellulose triacetate film was immersed in aquaregia for 15 minutes. The film was then soaked in cc. of 25 silvernitrate for 15 minutes. The film Was removed and soaked in a bathcontaining 10 cc. of 25% silver nitrate in a 150 cc. solution of 15grams per liter hydroquinone and one gram per liter gum arabic for 5minutes. The film was removed from this solution after 5 minutes, washedwith distilled water and then soaked in identical solution of silver andhydroquinone for 5 minutes. The resulting silver coated membrane wastested and found to be flexible and a good electrical conductor.

Example 2 A silver coated membrane prepared in accordance with Example 1was immersed in an electrodeposition bath containing iron ions. Thesilver coated film was the cathode in the electrodeposition bath. Theiron was electrically deposited upon the silver coating of the film. Theiron coated film was removed from the bath. The

film was found to have a continuous coating of iron thereon and testingproved the iron coating to be firmly adhered to the cellulose triacetatefilm.

Example 3 A silver coated film prepared in accordance with Example 1 wasimmersed in a 0.02 molar solution of gold chloride for 20 minutes. Thegold during this time replaced the silver of the silver coating therebyproviding a gold coated film. The film was removed from the gold bathand tested. The test determined that the metal coating was firmlyadhered to the cellulose triacetate film and was electricallyconductive.

Example 4 A 100 square inch cellulose triacetate film was immersed inaqua regia for 15 minutes. The film was then soaked in 150cc. of 25%silver nitrate for 15 minutes. The film was then removed and soaked in abath containing 10 cc. of 25 silver nitrate in a 150 cc. solution of 15grams per liter hydroquinone and 1 gram per liter gum arabic for 6minutes. The film was removed from this solution, Washed with distilledwater and then soaked in identical solution for minutes. After thesecond soaking the film was removed, washed with distilled water andsoaked in a 50 cc. solution of 0.01 molar gold chloride. The film wasremoved from the gold chloride bath and washed with 50 cc. of NH OH andin 50 cc. of distilled water. The ammonia Washed film was then immersedin an aqueous solution of nickel chloride and sodium hypophosphite witha sodium acetate butter until a continuous layer of nickel was formed.The acetate film was left in the nickel bath for about 9 to 11 minutes.The nickel coated film thus produced Was tested and found to becontinuous, electrically conductive and firmly bound to the cellulosetriacetate film.

1 What is claimed is:

I. A process for metal coating cellulose triacetate plastic film whichcomprises the steps of 1 (a) contacting cellulose triacetate plasticfilm with aqua regia for about to 20 minutes, and

(b) soaking the film prepared in step (a) in a solu- 4 tion of a noblemetal salt and a reducing agent to deposit a continuous metal coating onsaid film. I 2. A process as defined by claim 1 wherein said reducingagent is selected from the group consisting of a hydrophosphite salt andhydroquinone.

3. A process as defined by claim 1 wherein said reducing agent ishydroquinone.

4. A process as defined by claim 1 wherein said noble metal salt issilver nitrate.

5. A process for metal coating cellulose triacetate film which comprisesthe steps of (a) contacting cellulose triacetate plastic film with aquaregia for about 10 to 20 minutes,

([1) soaking the film prepared in step (a) in a solution of a noblemetal salt and a reducing agent to deposit a continuous silver coatingon said film,

(c) soaking the film prepared in step (b) in a solution comprising about0.01 molar gold chloride to replace said silver coating with acontinuous coating of gold,

(d) washing the film prepared in step (c) with a solution comprising asilver ion complexing agent to remove excess silver ions, and

(e) emersing the film prepared in step (d) in a solution comprising areducing agent and a metal salt selected from the group consisting ofsilver salts and nickel salts to deposit a continuous metal coating onthe film.

6. A process as defined by claim 5 wherein said complexing agent isammonium hydroxide.

References Cited by the Examiner UNITED STATES PATENTS 1,529,712 3/1925Pfiifner 11760 2,278,722 4/ 1942 Loiseleur 117-47 2,430,581 11/1947Pessel 117-160 X 2,757,104 7/1956 Howes 117213 X 2,934,457 4/ 1960Elliott 117-47 3,179,575 4/1965 Dippel et al 1l7217 X 3,235,473 2/1966Le Duc 117-47 MURRAY KATZ, Primary Examiner.

1. A PROCESS FOR METAL COATING CELLULOSE TRIACETATE PLASTIC FILM WHICHCOMPRISES THE STEPS OF (A) CONTACTING CELLULOSE TRIACETATE PLASTIC FILMWITH AQUA REGIA FOR ABOUT 10 TO 20 MINUTES, AND (B) SOAKING THE FILMPREPARED IN STEP (A) IN A SOLUTION OF A NOBLE METAL SALT AND A REDUCINGAGENT TO DEPOSIT A CONTINOUS METAL COATING ON SAID FILM.